1. Field of the Invention
The invention relates to an optical amplifier and a process for the amplification of an optical signal.
2. Description of the Prior Art
It is known to produce induced emission from active laser substance by means of optical pumps. In this process, the light source generating the pump light is selected such that its emission spectrum coincides with the absorption spectrum of the active laser substance. In the case of a suitable selection of the light source, the electrons of the active laser substance are excited to an energy state above the upper laser level. From this level, after a successful occupancy inversion, the substance relaxes, for example, by phonon emission to the upper laser level. From the upper laser level photons are emitted by induced emission if photons of an optical signal are present to produce an external radiation field. The active laser substance passes into the lower laser level which, in the case of the 3-level lasers, is the ground state. In the case of 4-level lasers, the active laser substance is converted from the lower laser level to the ground state by phonon emission.
An optical amplifier operating according to the principle of the 3-level laser is known from Electronics Letters 23 (1987) pp. 1026-1028.
An optical signal propagates in an optical waveguide, which is doped with a rare-earth compound, in this case with an erbium compound. The fiber amplifier contains a dye laser as a pump light source, whose light passes through a fiber coupler into the optical waveguide, where it induces the active laser substance to induce emission if light of the optical signal to be amplified passes through the optical waveguide.
In addition to the induced emission, during the return of the active laser substance to the lower state, there is also a spontaneous emission of photons, which causes a noise in the signal.